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SubjectRe: irq lock inversion
Hello, Jiri.

Jiri Kosina wrote:
> Hmm, I wonder why I don't see this lockdep warning myself with
> head on 1836d9592, even though I have
>
> CONFIG_PROVE_LOCKING=y
> CONFIG_TRACE_IRQFLAGS=y
>
> ... ?

You need pcpu_mem_free() hit vfree() to trigger the warning by
allocating a lot of small percpu areas so that allocation map inside a
chunk becomes larger than 4k and then get extended once more.

> Anyway, how about something like this? (I can't verify myself that it even
> fixes the warning, as I don't see it for some odd reason)
>
> From: Jiri Kosina <jkosina@suse.cz>
> Subject: lockdep: avoid false positives about irq-safety
>
> Commit 403a91b1 ("percpu: allow pcpu_alloc() to be called
> with IRQs off") introduced this warning:
>
> =========================================================
> [ INFO: possible irq lock inversion dependency detected ]
> 2.6.32-rc5-tip-04815-g12f0f93-dirty #745
> ---------------------------------------------------------
> hub 1-3:1.0: state 7 ports 2 chg 0000 evt 0004
> ksoftirqd/65/199 just changed the state of lock:
> (pcpu_lock){..-...}, at: [<ffffffff81130e04>] free_percpu+0x38/0x104
> but this lock took another, SOFTIRQ-unsafe lock in the past:
> (vmap_area_lock){+.+...}
>
> and interrupts could create inverse lock ordering between them.
>
> This warning is bogus -- sched_init() is being called very early with IRQs
> disabled, and the irqsave/restore code paths in pcpu_alloc() are only for early
> init. The path can never be called from irq context once the early init
> finishes. Rationale for this is explained in changelog of the commit mentioned
> above.
>
> This problem can be encountered generally in any other early code running
> with IRQs off and using irqsave/irqrestore.
>
> Reported-by: Yinghai Lu <yhlu.kernel@gmail.com>
> Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Looks good to me. Ingo, what do you think?

Thanks.

--
tejun


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